Method of dimensionally stabilizing a paper-plastic laminate



May 26, 1964 l.. s. 'roBlAs ETAL METHOD 0F DIMENSIONALLY STABILIZING A PAPER-PLASTIC LAM-INATE Filed Sept. '7, 1960 mafia@ PAPE/e INVENTOR ATTORNEY United States Patent O 3,1?:%,S34 METHGD F DIMENSIONALLY STABILIZING A PAPER-PLAS'HC LAMINATE Lawrence S. Tobias, Valley Stream, and (Bv/en B. Mosher, Glens Falls, Nfi., assignors to international Paper Company, New York, N.Y., a corporation of New York Filed Sept. 7, 1960, Ser. No. 5,627 2 Ciaims. (Cl. 264-340) This invention relates in general to commercial papers such as bond, graph, index, chart and map papers and the like, and particularly to a new and useful dimensionally stable laminate formed from two outer layers of paper and an intermediate layer of a heat-scalable rigid plastic. This application is a continuation-in-part of copending application, Serial No. 502,496, tiled April 19, 1955, now abandoned.

Grdinary paper expands and contracts with changes in humidity of the surrounding atmosphere. It expands upon absorption of moisture from the atmosphere as humidity increases and contracts when moisture is given up to the atmosphere as humidity decreases. Since paper as formed on a Fourdrinier machine comprises fibres which are aligned in the machine direction more than in Vthe cross-machine direction, expansion and contraction of the finished sheet is usually greatest in the cross-machine direction.

Many end uses of paper require dimensional stability of the paper or characteristics of the paper which insure little or no change in dimensions due toV changes in temperature or humidity. s nan example, business tabulating cards are punched out at precise positions to record information which is later picked up by machine sensing devices. Any change in position of the punched out portions which may result from changes in the dimensions of the cards due to humidity or other changes will cause the registry of inaccurate information. Therefore, when paper is to be used'for any such end uses where the dimensions must remain constant, such as for scale maps, graphs, charts, etc., it is necessary to have a paper with excellent dimensional stability.

Heretofore in the manufacturing and drying of paper requiring excellent dimensional stability attempts have been made to achieve a paper which would not expand or contract to an excessive extent, but the results have not been satisfactory. Attempts have been made to laminate paper in which the direction of the fibres on various laminae were disposed angularly to one another, but this was costly due to the necessity of using sheeted paper instead of rolls of paper, and the decreased expansion and contraction is relatively small. Many other means for reducing dimensional changes have been tried, including increased beating of the stock, addition of materials at the beater, and impregnation of the paper with materials such as wax or other materials. All have the disadvantage of changing the characteristics of the paper and adding to the cost of its manufacture without materially changing dimensional characteristics.

In some instances the paper has been stored and used only in locations having controlled temperatures and humidity kept within a range which will not result in the expansion or contraction of the paper. But the control of temperature and humidity is costly and unsatisfactory.

3,l34,834 Patented May 26, 1964 ICC Paper and card stock manufactured in accordance with this invention, however, overcomes all the deficiencies mentioned above. The invention is based on the discovery that by sandwiching Vor interposing a heat-scalable plastic having qualities of resistance to water absorption and elongation, such as polystyrene, between two sheets of any suitable type of paper, which itself may be of poor dimensional stability, it is possible to achieve a paper laminate product which is very stable dimensionally and which will not be affected by change of temperature and/ or humidity to any great extent.

Accordingly, it is an object of this invention to provide a paper laminate product which has a high degree of dimensional stability.

lt is a further object of this invention to provide a laminated paper product comprising outside laminae of paper having only good or moderate natural dimensional stability, bonded by a core of heat-scalable plastic which acts to prevent expansion or contraction of the bonded product during changes of temperature or humidity.

lt is a still further object to provide as an article of manufacture an inexpensive, dimensionally stable paper comprising a polystyrene, acrylic resin, methacrylate or 1 other relatively rigid non-moisture-absorbent plastic,

heat-sealed between paper sheets.

The various features of novelty which characterize the invention are pointed out with particularly in the claims annexed to and forming a part of this specification. For a better understanding of this invention, its operating advantages and the specific objects attained by its use, reference should be had tothe accompanying drawing and descriptive matter in` which there is illustrated and described a preferred embodiment of the invention.

The sole ligure of the drawing is a cross-section of a laminated paper product constructed in accordance with this invention.

The paper shown in the drawings comprises two sheets of ordinary, generally lightweight paper and an inner core of a commercial polystyrene plastic or other similar non-water absorbent plastic heat-sealed together to form a laminated paper product having all the good characteristics of ordinary paper and being substantially dimensionally stable when exposed to changes in humidity.

Paper made in accordance with this invention is easily fabricated by feeding the two laminae of paper on the outside of a preformed plastic web between pressure rolls or by extruding hot plastic between two paper webs as they approach the nip of two pressure rolls.

The outside laminae of paper comprise any ordinary paper sheets chosen for the paper characteristics desired in an end product. For such uses as for charts, map and machine-sorted classification cards papers are chosen which will give the desired ink receptiveness and then are made to the desired rigidity, caliper and basis weight by varying the quantities of plastic and/or the caliper and weight of the paper laminae.

lt has been found that a paper laminate made in any of the weights or calipers desired when compared with ordinary papers used for the same purposes is far more dimensionally stable than any papers heretofore known. For exai iple, the following table indicates the machinedirection and cross-direction expansion and contraction of ordinary commercial circular at chart paper as cornpared to two variations of paper laminates constructed 8 in accordance with this invention. All paper weights are on a 24 x 36"-500 sheet ream basis.

All Papers 6 Mz'ls Caliper EXPANSION DUE TO CHANGE OF RELATIVE HUMIDITY FROM 20% TO 75% AT 73 F.

Fibre direction CONTRACTION DUE TO CHANGE OF RELATIVE IIUBIID- ITY FROM 75% TO 20% AT 73 F.

Ordinary commercial chart 75 Paper 95# basis weight No. 1 paper laminate Paper 29# basis weight Polystyrene 37# basis weight No. 2 paper laminate Paper 13# basis weight 0. 094 Polystyrene 69# basis weight From the above it can be seen that a paper laminate made in accordance with this invention expands and contracts substantially less than a similar type of ordinary commercial paper of the same caliper and basis weight. In fact, even though each of the paper laminates compared is in every way suitable for the same use as the commercial chart paper insofar as the other normal paper characteristics are concerned, it is as much as nine times better, percentagewise, in the cross direction and six times in the machine direction, as far as expansion and. contraction due to humidity change are concerned.

Referring to the above table it can be seen that expansion and contraction of the paper laminate can be reduced much further by the addition of more polystyrene while reducing the basis Weight of paper laminae proportionately in order to arrive at a 6 mil caliper paper of the same combined basis weight. Paper laminate No. l is a 95# material made up of approximately 37thb basis weight of polystyrene and 29# basis weight of each paper lamina. Laminate No. 2 is also a 95# material, but has an increased amount of polystyrene in the core (694i) and a proportionately lighter paper (Bit). Since the same type of paper is used in each laminate, each possesses Vsimilar paper characteristics. However, sheet No. 2 is more stable dimensionally due to the increased polystyrene.

As the heat-scalable lamina core the polystyrene holds the outer paper laminae against dimensional change due to moisture. The thermal coeilicient of expansion oi Dow tyron 475, a commercially available polystyrene, is, for instance, 4 to 4.5 X10-5 in./in./F. according to Vthe ASTM test method and its water absorption in 24, hours is 0.12% by weight, dry. Other plastics such as acrylic resin and methacrylate which are also moisture-resistant are satisfactory for use as a core.

Experiments have shown that, when making a card paper suitable for use as machine-sorted classification and Y information registry cards, laminating with a polystyrene core has reduced expansion and contraction of the card due to humidity change by as much as three times in the machine direction and nine times in the cross direction. This can be even bettered by using a greater amount of polystyrene as a core and proportionately reducing the amount of outer paper laminae.

Experiments have also shown that when polystyrene is heated, as for extrusion, it is normally molten. When it is then Vdeposited as a thin lm to be sealed between two webs of paper, it cools to a solid, i.e., plastic, film ,after contacting and becoming somewhat imbedded in the paper. ln changing from a hot liquid to a hot solid, polystyrene normally tends to shrink about 0.6%. But, when such a lm cools between, in contact with, and imbedded in two paper webs, this tendency is resisted and/ or prevented by the paper and the polystyrene is or remains under tension. Evidence of this opposition of forces can be gleaned, from the fact that, when only one of the outer paper laminae of the product of the invention is moistened, a curl therein results which places thewet lamina inside or on the concave side of the curl, instead of outside or on the convex side of the curl where it would be found if a simple piece of paper were substituted for the laminated product. .The direction of the curl indicates that the wetting of the outer paper lamina, while it may cause the individual paper libres to expand, ultimately serves only to weaken the structural resistance of the lamina to the compressing forces exerted by the polystyrene center.

Thus, an internal stress and strain is constructed or built-into the laminated products of the invention and, so long as this dynamic balance of forces favors the tendency of the polystyrene to contract, dimensional changes in the outer paper laminae, e.g., expansions caused by their absorption of moisture and later contractions caused by their loss of moisture, are held to a surprisingly low minimum, since polystyrene is a rigid plastic and does not absorb moisture.

The following table shows a comparison of the expansion and contraction of a relatively thin 31/2 mil roll meter type of paper presently used commercially as compared with the same caliper paper laminate suitable for the same use:

Both Papers 31/2 Ml'ls Caliper EXPANSION DUE TO CHANGE OF RELATIVE HUMIDITY FROM 20% T0 75% AT 73 F.

Fibre direction Cross Machine Percent Percent 40# roll-meter paper 0.70 0.32 40# roll-meter paper laminate- Paper i3# basis Weight 0.30 0.20 lolystyrene 22# basis weight CONTRACTION DUE T O CHANGE OF RELATIVE HUMID- ITY FROM 75% to 20% AT 73 F.

40# roll-meter paper 0.30 40# roll-meter paper laminate.- Paper 13# basis Weight 0. 31 0.20

Polystyrene 22# basis weight paper laminate when exposed to large quantities of water showed that when the laminate is immersed in a water bath and then blotted dry the paper actually shrinks slightly instead of expanding, as is the case with ordinary paper. However, aside from the unusualness of the phenomenon, it is clear that such a characteristic can have its disadvantages and that it would be preferable, if possible, to shrink the laminate beforehand wherever its contemplated use entails a highly humid atmosphere and nevertheless requires a high degree of dimensional stability.

Accordingly, it has been found that the dimensionally stable paper laminate of the present invention can be given a future resistance, i.e., a relative immunity, to shrinkage due to moisture absorption by means of the steps of immersing the entire laminate in water fora short time, of allowing'the thus wet laminate tor stand for a longer period, and of uniformly drying it at a relatively slow rate. The resulting sheet or web is at and exhibits a minor amount of expansion or contraction Next, test specimens of the laminated Yroll Were preconditioned in the 20 leveling tubes of a Neenah expansirneter at 75% relative humidity for 2% hours before initial measurements were made, in order to eliminate With changing humidity. The magnitude of this expan- 5 errors in the rst cycle. The instrument contains 20 sion or contraction is dependent, as it Was prior to the leveling tube holders through which air of controlled pre-shrinking or post treating series of steps, on the ratio humidity and temperature can be passed. In each holder of the physical strength of the two paper surfaces as a strip of paper can be suspended and measured for compared to the polystyrene center. The type of cellulength with a micrometer adjustment. Thus, the 20 lose bre in the paper is a small factor, but, as before, 10 samples could be run through the various humidity cycles the ratio of the total weight of paper to the total Weight at one time and changes in humidity could be measured of polystyrene is an important factor. Such ratio is, without moving the instrument or disturbing the samples. preferably, about 60% by weight of paper and 40% by The results of these tests are set forth below in Table I weight of the resin. and the results of similar tests made on avariety of Just how the pre-shrinking or the post treating of the laminated products made in accordance with the invendimensionally stable paper laminate of the invention iS tion, some of which were and some of which were not effected is not conclusively known, but it is believed that, post treated, i.e., water-immersed and dried, in accord- When the entire laminate is soaked in Water and allowed ance with the invention are set forth below in Table II. to stand in a wet condition for a period of time, the In such tables, Con means the percent contraction in stresses originally created in the polystyrene upon its the samples due to a change of the relative humidity contacting of and imbedding in the two paper Webs with from 80% to 25% each sample experienced during the which it is laminated are released. The polystyrene is tests, Ex. means the percent expansion in the samples then no longer under tension and it assumes its normal due to a change of the relative humidity from to dimensions. And, thereafter, if the entire lamination is 85% each experienced during the tests, M.D. means dried uniformly, at not too fast a rate, and, preferably, 25 that the measurement of expansion or contraction was under a slight tension, the paper on each side of the measured in the machine direction of the paper fibres, and polystyrene will, because of its water-weakened struc- C.D. means that the measurement of expansion or contural strength and despite the expansion of its individual traction was measured in the cross-machine direction of fibres due to water absorption, conform to the normal dithe paper fibres.

TABLE I Test Cycle No.

Con. Ex. Con. Ex. Con. Ex. Con. Ex. Con. Ex. Con. Ex. Con. Ex

M.D .21 .20 .21 .19 .20 .2o .2o .2o .20 .21 .20 .2o .20 .2o

C.D .35 .37 .35 .33 .34 .35 .34 .33 .34 .as .a3 .32 32 .32

mensions of the polystyrene center without creating new TABLE II stresses or strains'.

Preferably, the immersion bath of the post treating r Test @me N0 series of steps has a temperature of not higher than about 4" 65 F. or lower than room temperature, the time of imgamme 1 2 3 .1 5 mersion is from about 3 to l0 seconds, the standing period is in the range of about l5 minutes to 24 hours, and the EX @on EX COIL Ex C011 Ex C011 EL @on time of drying, if it is done in a tunnel dryer having a temperature in the range of from 100 F. to 225 F., is 50 A g1g 07 11 1 12 12 1.1 11 11 .12 13 from about 20 to 50 seconds. Thus, for example, in a D- 13 28 25 -23 -28 -29 -28 '27 -27 -29 recent trial, polystyrene (DOW Styron 475) was extruded AT" 11%).' i171 1311 ig Iig Iig I Iig 23g lig Iig hh h7h h hhhhhhh h 2h hh hh hhhhhh hhh hhwh a :a a a a a a1 a a :a downto 2 mds, S0 that, when maklng Contact Slmul- 55 B-T--. Mp. 16 .19 11 .17 .17 .19 .17 .11 .17 .19 hhhhhhhh .Whh hwh hh hh hhh hhh hhhh hhhhh whhh. hhhh a a a :a a a a a a Wle feeding flOm each Sld Of the (lle lIllO the nlp 0f C D 15 28 27 29 29 2g 2g 27 29 30 pressure rolls, a 95 lb. per ream laminated product con- 0-T gij? (75 :g tammg lb. per ream of the polystyrene and havrg D 19111131 gg gg 2 gj a caliper of 6.0 mils resulted. The lamination from e -3 nip proceeded straight down to the bottom of the ma- D T" i; :g2 :g2 :gg gg chine before contacting a turning roll, so that the polystyrene could set in a at plane before being trimmed and wound on the Winder. Next, the laminated roll was In Table II, Samples A and A-T were made from 30 1b. run through a bath of water at a speed assuring an apper ream bleached kraft paper and Dow Styron 475 and proximately 3 second immersion of the laminate and the 65 had a caliper of 7.3 mils; Samples B and B-T were made excess of water was squeezed off between a rubber presfrom 2S lb. per ream register paper and Dow Styron 475 sure roll and a chrome chill roll, so that the laminated and had a caliper of 6.9 mils; Samples C and C T were paper on the roll had, when rewound, an approximately made from 29 lb. per ream chart paper and Dow Styron 17% moisture content. Immediately after rewinding, the 475 and had a caliper in the range of 5.8 to 6.4 mils; and, laminated roll was Wrapped in a polyethylene bag and Samples D and D-T were vmade from 27 lb. per ream waterproof paper and stored for about 4 hours. Next, chart paper and Dow Styron 475 and had a caliper of the laminated roll was run through a tunnel dryer at 5.9 mils. The letter T symbolizes the fact that the slow speed countercurrently to forced hot air supplied sample in question was given the water immersion and to both outer paper larninae. The temperature in vthe drying post treatment before the tests were made on the tunnel was held below F. 75 Neenah Expansimeter.

What is claimed is:

1. In the production of dimensionally stable paper having ink `receptiveness and adapted for use in the form of tabulating cards, maps, charts, and graphs, a first step of immersing in Water a paper laminate consisting .of two outer sheets of paper heat-sealed to a polystyrene core under tension, the core being from about 37% to 73% vby weight of the laminate, at from about room temperature to 65 F. for from about 3 to 10 seconds; a second step of allowing the wet paper laminate to stand for from about 1.5 minutes to 24 hours; and, a third step .of drying lthe resulting paper laminate `in a tunnel dryer at from about 100 F. to 225 F. for from about 20 to 5'0 seconds.

2. A process in accordance with claim 1 in which they Weight of the polystyrene core is about 40% ofthe Weight of the laminate.

References Cited-in-the le of this patent UNITED STATES PATENTS Parker Aug. 26, 1930 Blum May 23, 1933 lCorcoran Oct. 16, 1934 Robertson Ian. 2, 1940 Cowgill May 29, 1951 Munro Oct. 25, 1955 Robinson et a1 Dec. 3, 1957 

1. IN THE PRODUCTION OF DIMENSIONALLY STABLE PAPER HAVING INK RECEPTIVENESS AND ADAPTED FOR USE IN THE FORM OF TABULATING CARDS, MAPS, CHARTS, AND GRAPHS, A FIRST STEP OF IMMERSING IN WATER A PAPER LAMINATE CONSISTING OF TWO OUTER SHEETS OF PAPER HEAT-SEALED TO A POLYSTRYRENE CORE UNDER TENSION, THE CORE BEING FROM ABOUT 37% TO 73% BY WEIGHT OF THE LAMINATE, AT FROM ABOUT ROOM TEMPERATURE TO 65*F. FOR FROM ABOUT 3 TO 10 SECONDS; A SECOND STEP OF ALLOWING THE WET PAPER LAMINATE TO STAND FOR FROM ABOUT 15 MINUTES TO 24 HOURS; AND, A THIRD STEP OF DRYING THE RESULTING PAPER LAMINATE IN A TUNNEL DRYER AT FROM ABOUT 100*F. TO 225*F. FOR FROM ABOUT 20 TO 50 SECONDS. 